In the classical Koehler illumination principle, a light source situated at a finite distance from a condenser is imaged by a field lens directly into the aperture diaphragm of this condenser, and the field diaphragm disposed behind the field lens is imaged by the condenser into the specimen plane (Beyer-Riesenberg: Handbuch der Mikroskopie, 3rd Edition, Berlin 1988, p. 145). Different distances between the light source and field diaphragm and the specimen plane (optical transmission distance) require a particular sizing and correction of the imaging systems for each specific microscope. In the case of great transmission lengths this result in lens diameters for a given focal length which are no longer usable.
A Variosystem (DE-PS 26 35 142) is known which has a finite light path and produces real intermediate images of the light source and field diaphragm. This Variosystem, however, is not economically feasible for simple educational microscopes.
In another known illumination system according to DE-PS 28 46 056, in addition to different condenser systems, different aperture diaphragms lying in different planes are provided, and therefore, again, the illumination of specimen fields and apertures calls for considerable complexity.
For the illumination of specimen fields and apertures of different sizes, a known illumination system according to DE-PS 31 13 843 provides two interchangeable relay lens systems which receive an intermediate image of the light source projected by the field lens at a finite distance and image it at a finite distance (System 1) or at an infinite distance (System 2). Aside from the increased expense involved in the use of relay lens systems, the telecentric imaging of the light source in the specimen field when the magnification of the microscope objective is low leads as a rule to shading of the illuminated specimen field by the imaging optical systems that follow.
Microscopes are also known in which, for the advantageous placement of optical components such as polarizers or interference contrast prisms in the telecentric light path, the luminous field diaphragm is imaged in infinity (Jenaer Rundschau 28 (1953) 2 61 . . . 62). For light-source imaging at infinity, however, a specially constructed intermediate imaging system is required for each of the microscope illuminators that can be used, due to the great optical transmission length.
It is common to all of the above-mentioned illumination systems that they are each intended for a specific structure of the microscope with a given optical transmission length, and it is not possible to employ them using the same components for microscopes of different optical transmission length.